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use crate::{
fmt,
iter::FusedIterator,
mem::{self, MaybeUninit},
ptr,
};
/// An iterator over the mapped windows of another iterator.
///
/// This `struct` is created by the [`Iterator::map_windows`]. See its
/// documentation for more information.
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
pub struct MapWindows<I: Iterator, F, const N: usize> {
f: F,
inner: MapWindowsInner<I, N>,
}
struct MapWindowsInner<I: Iterator, const N: usize> {
// We fuse the inner iterator because there shouldn't be "holes" in
// the sliding window. Once the iterator returns a `None`, we make
// our `MapWindows` iterator return `None` forever.
iter: Option<I>,
// Since iterators are assumed lazy, i.e. it only yields an item when
// `Iterator::next()` is called, and `MapWindows` is not an exception.
//
// Before the first iteration, we keep the buffer `None`. When the user
// first call `next` or other methods that makes the iterator advance,
// we collect the first `N` items yielded from the inner iterator and
// put it into the buffer.
//
// When the inner iterator has returned a `None` (i.e. fused), we take
// away this `buffer` and leave it `None` to reclaim its resources.
//
// FIXME: should we shrink the size of `buffer` using niche optimization?
buffer: Option<Buffer<I::Item, N>>,
}
// `Buffer` uses two times of space to reduce moves among the iterations.
// `Buffer<T, N>` is semantically `[MaybeUninit<T>; 2 * N]`. However, due
// to limitations of const generics, we use this different type. Note that
// it has the same underlying memory layout.
struct Buffer<T, const N: usize> {
// Invariant: `self.buffer[self.start..self.start + N]` is initialized,
// with all other elements being uninitialized. This also
// implies that `self.start <= N`.
buffer: [[MaybeUninit<T>; N]; 2],
start: usize,
}
impl<I: Iterator, F, const N: usize> MapWindows<I, F, N> {
pub(in crate::iter) fn new(iter: I, f: F) -> Self {
assert!(N != 0, "array in `Iterator::map_windows` must contain more than 0 elements");
// Only ZST arrays' length can be so large.
if mem::size_of::<I::Item>() == 0 {
assert!(
N.checked_mul(2).is_some(),
"array size of `Iterator::map_windows` is too large"
);
}
Self { inner: MapWindowsInner::new(iter), f }
}
}
impl<I: Iterator, const N: usize> MapWindowsInner<I, N> {
#[inline]
fn new(iter: I) -> Self {
Self { iter: Some(iter), buffer: None }
}
fn next_window(&mut self) -> Option<&[I::Item; N]> {
let iter = self.iter.as_mut()?;
match self.buffer {
// It is the first time to advance. We collect
// the first `N` items from `self.iter` to initialize `self.buffer`.
None => self.buffer = Buffer::try_from_iter(iter),
Some(ref mut buffer) => match iter.next() {
None => {
// Fuse the inner iterator since it yields a `None`.
self.iter.take();
self.buffer.take();
}
// Advance the iterator. We first call `next` before changing our buffer
// at all. This means that if `next` panics, our invariant is upheld and
// our `Drop` impl drops the correct elements.
Some(item) => buffer.push(item),
},
}
self.buffer.as_ref().map(Buffer::as_array_ref)
}
fn size_hint(&self) -> (usize, Option<usize>) {
let Some(ref iter) = self.iter else { return (0, Some(0)) };
let (lo, hi) = iter.size_hint();
if self.buffer.is_some() {
// If the first `N` items are already yielded by the inner iterator,
// the size hint is then equal to the that of the inner iterator's.
(lo, hi)
} else {
// If the first `N` items are not yet yielded by the inner iterator,
// the first `N` elements should be counted as one window, so both bounds
// should subtract `N - 1`.
(lo.saturating_sub(N - 1), hi.map(|hi| hi.saturating_sub(N - 1)))
}
}
}
impl<T, const N: usize> Buffer<T, N> {
fn try_from_iter(iter: &mut impl Iterator<Item = T>) -> Option<Self> {
let first_half = crate::array::iter_next_chunk(iter).ok()?;
let buffer = [MaybeUninit::new(first_half).transpose(), MaybeUninit::uninit_array()];
Some(Self { buffer, start: 0 })
}
#[inline]
fn buffer_ptr(&self) -> *const MaybeUninit<T> {
self.buffer.as_ptr().cast()
}
#[inline]
fn buffer_mut_ptr(&mut self) -> *mut MaybeUninit<T> {
self.buffer.as_mut_ptr().cast()
}
#[inline]
fn as_array_ref(&self) -> &[T; N] {
debug_assert!(self.start + N <= 2 * N);
// SAFETY: our invariant guarantees these elements are initialized.
unsafe { &*self.buffer_ptr().add(self.start).cast() }
}
#[inline]
fn as_uninit_array_mut(&mut self) -> &mut MaybeUninit<[T; N]> {
debug_assert!(self.start + N <= 2 * N);
// SAFETY: our invariant guarantees these elements are in bounds.
unsafe { &mut *self.buffer_mut_ptr().add(self.start).cast() }
}
/// Pushes a new item `next` to the back, and pops the front-most one.
///
/// All the elements will be shifted to the front end when pushing reaches
/// the back end.
fn push(&mut self, next: T) {
let buffer_mut_ptr = self.buffer_mut_ptr();
debug_assert!(self.start + N <= 2 * N);
let to_drop = if self.start == N {
// We have reached the end of our buffer and have to copy
// everything to the start. Example layout for N = 3.
//
// 0 1 2 3 4 5 0 1 2 3 4 5
// ┌───┬───┬───┬───┬───┬───┐ ┌───┬───┬───┬───┬───┬───┐
// │ - │ - │ - │ a │ b │ c │ -> │ b │ c │ n │ - │ - │ - │
// └───┴───┴───┴───┴───┴───┘ └───┴───┴───┴───┴───┴───┘
// ↑ ↑
// start start
// SAFETY: the two pointers are valid for reads/writes of N -1
// elements because our array's size is semantically 2 * N. The
// regions also don't overlap for the same reason.
//
// We leave the old elements in place. As soon as `start` is set
// to 0, we treat them as uninitialized and treat their copies
// as initialized.
let to_drop = unsafe {
ptr::copy_nonoverlapping(buffer_mut_ptr.add(self.start + 1), buffer_mut_ptr, N - 1);
(*buffer_mut_ptr.add(N - 1)).write(next);
buffer_mut_ptr.add(self.start)
};
self.start = 0;
to_drop
} else {
// SAFETY: `self.start` is < N as guaranteed by the invariant
// plus the check above. Even if the drop at the end panics,
// the invariant is upheld.
//
// Example layout for N = 3:
//
// 0 1 2 3 4 5 0 1 2 3 4 5
// ┌───┬───┬───┬───┬───┬───┐ ┌───┬───┬───┬───┬───┬───┐
// │ - │ a │ b │ c │ - │ - │ -> │ - │ - │ b │ c │ n │ - │
// └───┴───┴───┴───┴───┴───┘ └───┴───┴───┴───┴───┴───┘
// ↑ ↑
// start start
//
let to_drop = unsafe {
(*buffer_mut_ptr.add(self.start + N)).write(next);
buffer_mut_ptr.add(self.start)
};
self.start += 1;
to_drop
};
// SAFETY: the index is valid and this is element `a` in the
// diagram above and has not been dropped yet.
unsafe { ptr::drop_in_place(to_drop.cast::<T>()) };
}
}
impl<T: Clone, const N: usize> Clone for Buffer<T, N> {
fn clone(&self) -> Self {
let mut buffer = Buffer {
buffer: [MaybeUninit::uninit_array(), MaybeUninit::uninit_array()],
start: self.start,
};
buffer.as_uninit_array_mut().write(self.as_array_ref().clone());
buffer
}
}
impl<I, const N: usize> Clone for MapWindowsInner<I, N>
where
I: Iterator + Clone,
I::Item: Clone,
{
fn clone(&self) -> Self {
Self { iter: self.iter.clone(), buffer: self.buffer.clone() }
}
}
impl<T, const N: usize> Drop for Buffer<T, N> {
fn drop(&mut self) {
// SAFETY: our invariant guarantees that N elements starting from
// `self.start` are initialized. We drop them here.
unsafe {
let initialized_part: *mut [T] = crate::ptr::slice_from_raw_parts_mut(
self.buffer_mut_ptr().add(self.start).cast(),
N,
);
ptr::drop_in_place(initialized_part);
}
}
}
#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
impl<I, F, R, const N: usize> Iterator for MapWindows<I, F, N>
where
I: Iterator,
F: FnMut(&[I::Item; N]) -> R,
{
type Item = R;
fn next(&mut self) -> Option<Self::Item> {
let window = self.inner.next_window()?;
let out = (self.f)(window);
Some(out)
}
fn size_hint(&self) -> (usize, Option<usize>) {
self.inner.size_hint()
}
}
// Note that even if the inner iterator not fused, the `MapWindows` is still fused,
// because we don't allow "holes" in the mapping window.
#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
impl<I, F, R, const N: usize> FusedIterator for MapWindows<I, F, N>
where
I: Iterator,
F: FnMut(&[I::Item; N]) -> R,
{
}
#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
impl<I, F, R, const N: usize> ExactSizeIterator for MapWindows<I, F, N>
where
I: ExactSizeIterator,
F: FnMut(&[I::Item; N]) -> R,
{
}
#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
impl<I: Iterator + fmt::Debug, F, const N: usize> fmt::Debug for MapWindows<I, F, N> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("MapWindows").field("iter", &self.inner.iter).finish()
}
}
#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
impl<I, F, const N: usize> Clone for MapWindows<I, F, N>
where
I: Iterator + Clone,
F: Clone,
I::Item: Clone,
{
fn clone(&self) -> Self {
Self { f: self.f.clone(), inner: self.inner.clone() }
}
}